Photosynthesis converts energy from the sun, plus carbon dioxide (CO2) and water (H2O) into carbohydrates (such as glucose, C6H12O6) used for plant growth and oxygen (O2). Photosynthesis takes place in the chloroplasts, specifically using pigments such chlorophyll and carotenoids. Photons that have a wavelength between 400 and 700 nanometers (nm) provide the energy for photosynthesis. More specifically, light is mostly absorbed by chlorophyll in the blue (400 nm-500 nm) and red (600 nm-700 nm) regions (i.e. wavelengths) of the light spectrum and by carotenoids in the blue region.
Daily light integral (DLI) refers to the number of photosynthetic light particles, or photons, received during one day in a particular location and area. The DLI specifically refers to the amount of light received in 1 m2 per day. It is measured in mol·m−2·d−1, i.e., moles of light (mol) photons per square meter (m−2) per day (d−1), with each mol consisting of 6.02×1023 photons of light.
The maximum natural DLI is about 60 mol·m−2·d−1 and occurs outdoors on a cloudless day in the summer when the photoperiod is long. The DLI outdoors may be less than 5 mol·m−2·d−1 in the winter on a dark, cloudy, short day in the northern part of the United States or Canada. Inside a greenhouse, the structure and glazing materials commonly reduce light transmission by 35-50 percent. In a greenhouse, values seldom exceed 25 mol·m−2·d−1 because of greenhouse glazing materials and superstructure, the season (which affects the sun's angle), cloud cover, day length (photoperiod), shading, and greenhouse obstructions, such as hanging baskets. Therefore, the DLI inside a greenhouse in the United States may be from about 5 to 30 mol·m−2·d−1, depending upon location, season and greenhouse configuration.
The DLI that is needed to grow high-quality plants depends upon the crop, but a common target minimum DLI inside a greenhouse is 10-12 mol·m−2·d−1. Plant quality generally increases as the average DLI increases. In particular, as the DLI increases, branching, rooting, stem thickness and flower number increase.
When the DLI is low outdoors, growers are wise to maximize the amount of natural light that can reach their crops. For example, in greenhouses shading may be removed, glazing may be cleaned and overhead obstructions may be kept to a minimum. If such measures are impractical or insufficient, the DLI may be increased by supplemental lighting.
While beneficial, supplemental lighting is not without risks. Supplemental lighting outside certain wavelengths may limit productivity. Excessive supplemental lighting may harm plants. Heat from excessive lighting can be detrimental. Photosynthesis and other plant growth processes shut down when the environmental and tissue temperature gets high enough from heat energy that comes with the light. At that point all the water taken up by the plant is used to cool the plant tissue. Plants receiving excessive amounts of light thus dry up, become bleached through the destruction of chlorophyll, and may display other symptoms of excessive stress. At full intensity, supplemental lighting may subject plants to lighting that exceeds the plants' photosynthetic capacity. This may lead to reversible and, eventually, irreversible photoinhibition. While reversible photoinhibition is a temporary protective mechanism, irreversible photoinhibition permanently damages the light-harvesting reactions of the photosynthetic apparatus caused by excess light energy trapped by chloroplasts.
In nature, nighttime substantially reduces risk of heat stress and photoinhibition. In greenhouses with supplemental lighting, lights must be turned off to reduce risk of photoinhibition, leaving all plants simultaneously in a state of darkness.
What is needed is an efficient scalable growing system, that mimics natural daylight and nighttime conditions to ensure a DLI suitable for optimal plant growth without causing heat stress. The system should expose some plants to nighttime darkness while others are exposed to daylight illumination, irrigation and fertigation. Preferably, the system includes vertically oriented conveyors for high density growing that uses available vertical space. The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.